PTFE has excellent chemical resistance, heat resistance, and electrical insulating properties as well as properties such as self-lubricating properties and non-adhesive properties, and thus has been widely used in the fields of daily life as well as the industrial field. On the other hand, these properties mean difficulty in processing of PTFE. In other words, PTFE, though classified as a thermoplastic resin, is different from common plastics, such as polyethylene and vinyl chloride resin, and exhibits no flowability even when heated to 327° C. or higher where PTFE is in a non-crystalline state, and thus processes such as screw extrusion, injection molding, and roll forming in a heated state cannot be applied. Furthermore, even if one tries to prepare a PTFE solution and apply it to the surface of a substrate or coat the substrate, it is difficult to prepare the PTFE solution because there is no appropriate solvent, and even if one tries to bond a PTFE formed product to a target substrate, an adhesive that allows for a direct bond has not been discovered yet. In addition, heat fusion of PTFE and PTFE or PTFE and other resins, though possible, requires a high pressure, and PTFE cannot be easily bonded unlike other plastics.
Previously developed methods of processing PTFE are similar to methods of powder metallurgy. Examples include press-forming PTFE at about room temperature and sintering the press-formed product by heating it to 327° C. or higher; further forming this (sintered body), for example, by machine cutting or heat coining; extrusion-molding a mixture of PTFE powder and a liquid lubricant using a ram-type extruder, and then drying and sintering the extrudate for production of pipes and tubes or wire coating; and coating a substrate with an aqueous suspension of PTFE resin, for example, by application or dipping, and then sintering the coated substrate.
When PTFE is processed into an ultrafine fiber (also referred to as “nanofiber”), electrospinning (also referred to as “electrodeposition” or “electrostatic spinning”) as disclosed in Patent Documents 1 to 4 and 7 to 10 or stretching methods as disclosed in Patent Documents 5 and 6 can be used.
Patent Document 1 discloses a method of producing a nanofiber as shown in FIG. 1 by spinning from a PTFE dispersion containing polyethylene oxide (PEO) by electrospinning, and then removing PEO simultaneously with sintering. According to the production method disclosed in Patent Document 1, fiber diameter, basis weight, and the like can be controlled by selecting solution conditions and spinning conditions, and fibers can be oriented by using a special apparatus. Further, materials can be easily composited, and nanofibers having a high aspect ratio and a uniform diameter can be produced. However, the fiber diameter is about 500 nm at a minimum.
Patent Document 2 discloses a nonwoven fabric in which microfibers with a diameter of 0.001 to 1 μm formed by electrostatic spinning and ultrafine fibers with a diameter of 2 to 25 μm formed by melt blowing coexist, and polyvinylidene fluoride (PVDF) is given as an example of a fluorine resin constituting the microfibers formed by electrostatic spinning (paragraph [0019]).
Patent Document 3 discloses an apparatus that is able to prevent interference between adjacent nozzles and, in addition, to deposit different polymer solutions simultaneously in a multi-nozzle electrodeposition method (electrospinning method). In a polymer web produced using this apparatus, fibers are not joined together, although they may be entangled with each other.
Patent Document 4 discloses a production method comprising the step of feeding a polymer solution obtained by dissolving a polymer in a solvent into one rotary container at the circumference of which a plurality of small holes with different diameters are formed or a plurality of rotary containers that are concentrically united, and the step of electrifying the polymer solution that flows out of the small holes upon rotation of the rotary container and stretching the polymer solution that flows out of the small holes by means of centrifugal force and electrostatic explosion due to evaporation of the solvent, thereby forming a nanofiber comprising the polymer. According to this production method, a polymer web can be produced which is formed by mixing or laminating various nanofibers with different physical properties and depositing the mixture or laminate, but there are no embodiments where the fibers with different physical properties are joined together.
Patent Document 5 discloses a method of producing a porous structure (FIG. 2), in which an unsintered tetrafluoroethylene resin (i.e., PTFE) mixture containing a liquid lubricant is formed by extrusion and/or rolling, stretched in the unsintered state in at least one direction, and then heated to about 327° C. or higher. The unsintered tetrafluoroethylene resin tends to form a fine fibrous structure when subjected to shear forces: e.g., when extruded though a die during the extrusion process, when calendered under a roll, or when vigorously stirred. The resin containing a liquid lubricant is more easily fibrillized (page 2, right column, lines 9 to 13). As shown in FIG. 2, thick massive nodes (also referred to as “knots”) and thin fibrous fibrils coexist, the nodes having a fiber diameter of several micrometers to 1 μm, the fibrils having a fiber diameter of about 100 nm. According to the production method disclosed in Patent Document 5, fibers can be oriented by stretching and heating.
Patent Document 6 discloses a polytetrafluoroethylene porous body having a fine fibrous structure comprising fibers and knots connected to each other by the fibers, and this PTFE porous body has reticularly and three-dimensionally continuous short-fiber sections. According to the method of producing the PTFE porous body in Patent Document 6, unsintered PTFE powder and a liquid lubricant are mixed first and formed into a desired shape, for example, by extruding or rolling. The formed product obtained, from which the liquid lubricant may or may not be removed, is then stretched in at least one direction to form the PTFE porous body having a fine fibrous structure comprising fibers and knots connected to each other by the fibers.
Patent Document 7 discloses a method of producing a fiber sheet comprising uniaxially reoriented fibers by forming a fiber assembly from a spinning solution containing polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (paragraph [0016]), or the like by electrostatic spinning, and then stretching the fiber assembly in one direction.
Patent Document 8 discloses a method of producing a continuous filament composed of nanofibers with a diameter of, preferably, 500 nm or less through a continuous process using an electrospinning technique. Poly (ε-caprolactone) polymer (Example 1), polyurethane resin (Example 2), and nylon 6-resin (Example 3) are given as specific examples of polymers constituting the nanofibers.
Patent Document 9 discloses a method of producing a continuous filament composed of nanofibers with a diameter of, preferably, 500 nm or less from a polymer spinning solution containing a nylon resin (e.g., Example 1) through a continuous process using an electrostatic spinning technique.
Patent Document 10 discloses a wet-laid nonwoven fabric, wherein a wet-laid fiber web comprising a wholly aromatic polyamide fiber having fibrils and a polyester resin fiber is irradiated with infrared rays under no pressure, whereby the wholly aromatic polyamide fiber is fixed by the polyester resin solidified in a non-fibrous state at its fiber intersection. There is described that PTFE can be used in place of the wholly aromatic polyamide fiber (paragraph [0032]), but this is not specifically demonstrated in Examples or anywhere else.
Anyway, for fluororesin fiber sheets comprising fluororesin fibers, there is room for further improvement in sheet-like filters having both excellent properties (e.g., water repellency, heat resistance, chemical resistance, and sound permeability) of PTFE and a high specific surface area.
By the way, there is proposed the use of a hydrophilized microporous membrane comprising a crystalline polymer including PTFE as a filter for filtration or sterilization (Patent Document 11).
Commonly known methods of hydrophilization include irradiation with ultraviolet laser or ArF laser and chemical etching with a metallic sodium-naphthalene complex (Patent Document 12).
Further, in Patent Documents 11 and 13, hydrophilicity of a membrane is improved by employing a hydrophilic treatment in which the membrane is coated with polyvinyl alcohol (PVA), which is then crosslinked using an epoxy compound.
However, there remains room for further improvement in filtering performance of the filters for filtration disclosed in Patent Documents 11 to 13.